1. Field of the Invention
The present invention relates generally to a control device for a vehicle, and more particularly to a control device for a vehicle for cooperatively controlling fuel cut at deceleration and the engagement of a lock-up clutch.
2. Description of the Related Art
In a general vehicular engine, the supply of fuel to the engine is cut off at deceleration under given conditions, i.e., so-called fuel cut is performed, so as to improve the fuel economy. In a deceleration fuel cut method, a fuel cut execution decision engine speed and a return decision engine speed are set, and a fuel cut execution condition is determined according to a throttle valve opening angle. If the fuel cut execution condition is satisfied, it is determined whether or not the engine speed is higher than the fuel cut execution decision engine speed. If the engine speed is higher than the fuel cut execution decision engine speed, the supply of fuel to the engine is cut off after the elapse of a predetermined time from the time of satisfaction of the fuel cut execution condition.
When the engine speed becomes lower than the return decision engine speed after the execution of the fuel cut, the supply of fuel to the engine is restored, and the fuel cut execution decision engine speed is increased by a predetermined value for a predetermined time period from the time of restoration of the fuel supply. Thus, the fuel cut execution decision engine speed is increased by a predetermined value for a predetermined time after the supply of fuel to the engine is restored. Accordingly, it is possible to prevent the occurrence of hunting of engine speed such that the fuel cut and the fuel supply restoration are repeated after the fuel supply is restored.
Further, in general, an automatic transmission having a torque converter includes a lock-up clutch for directly connecting an output shaft of the engine and an input shaft of the automatic transmission in the condition where the gear position of the automatic transmission is a predetermined gear position and the rotational speed of the engine is greater than or equal to a predetermined rotational speed, in order to improve the fuel economy. In the case that such an automatic transmission with a lock-up clutch is combined with an engine having a fuel cut mechanism, various proposals have been made on the relation between an engine speed at the time of shifting to a lock-up condition and an engine speed at the time of restoration of the fuel supply after the fuel cut, in order to further improve the fuel economy as maintaining the drivability.
As mentioned above, to prevent the engine hunting due to the fuel cut during deceleration running, the fuel cut is not permitted until the engine speed becomes greater than the normal fuel cut execution decision engine speed in the condition where the accelerator pedal is kept undepressed after restoration from the fuel cut. Such control has a problem such that when the vehicle is accelerated by a running resistance and an acceleration of gravity from a very low vehicle speed without depression of the accelerator pedal on a downhill road in particular, engine brake by the fuel cut is not produced until reaching a high vehicle speed, causing a reduction in drivability due to a sense of free running. Further, the frequency of braking operations is increased to maintain the vehicle speed, resulting in an increase in brake load.